Changes of physical match performance after the COVID-19 lockdown in professional soccer players according to their playing position

The aim of this study was to evaluate the impact of the pandemic lockdown, which included training cessation and home-based training, on physical performance in professional soccer players from the Polish Ekstraklasa according to their playing position. The research was based on 3137 players’ individual match performance observations during the 2019/2020 season divided into before (26 matchdays) and after (11 matchdays) lockdown periods. The data were collected using the computerised multiple-camera optical tracking system TRACAB (ChryronHego VID, New York, NY) with a sampling frequency of 25 Hz. Independently of playing position, a significant (p < 0.001) reduction in season resumption metrics of total distance (-0.9%), jogging (-1.6%), running (-3.4%) and high-speed running (-2.5%), with a contemporaneous increase in walking distance (1.4%), was noted in relation to pre-lockdown performance. A reduction in high-speed running was observed in central defenders (p < 0.05), central midfielders (p < 0.01) and forwards (p < 0.05). No significant decrease in sprinting or maximal running velocity was observed. The COVID-19 lockdown negatively influenced the physical performance in professional soccer players.


INTRODUCTION
COVID-19 became a worldwide health issue in December 2019.
Three months later, on 11 March 2020, the World Health Organization (WHO) declared COVID-19 a pandemic [1]. After this decision, numerous countries implemented restrictions and limitations covering many fields, including sport. Consequently, soccer federations across the world decided to suspend the competition at all levels.
Afterwards, the procedures allowing for season resumption were prepared in cooperation with medical committees [2][3][4][5]. The typical training sessions for professional soccer players were not allowed as well. In this context, coaches prepared home-based training programmes to maintain the level of players' physical fitness until the resumption of regular training sessions and official matches. These programmes consisted of such activities as aerobic running, using stationary devices (bike or treadmill), and strength exercises using body weight and small weights.
In European elite soccer leagues, the season is typically divided into three parts: the preseason period, competitive period,

Changes of physical match performance after the COVID-19 lockdown in professional soccer players according to their playing position
It is well established within research literature that physical demands differ across playing positions during official soccer matches [20][21][22]. Thus, the unplanned off-season period may result in changes in ability of position-specific match performance. However, to the authors' knowledge, none of the previous studies have analysed the changes in match running performance caused by  lockdown according to position on the pitch. Therefore, the aim of this study was to evaluate the potential differences in the physical performance of professional soccer players and examine which playing positions were most affected by the COVID-19 lockdown. Based on previous scientific literature [17][18][19]23], we hypothesized that professional soccer players would decrease their match running performance after the pandemic lockdown. Furthermore, these physical performance reductions could depend on the playing position.  [11,12]. Rampinini et al. [13] reported that home-based training (4-5 aerobic sessions +2 or 3 strength training sessions a week) performed during COVID-19 lockdown had a positive effect on players' aerobic fitness, although anaerobic power levels were not maintained. Similar results were reported by Grazioli et al. [14] in a sample of Brazilian professional soccer players. Moreover, Parpa and Michaelides [15] confirmed that despite the challenges of the pandemic, properly planned and performed home-based training may significantly improve the level of physical fitness in professional soccer players. In contrast, Albuquerque-Freire et al. [16] found a significant decline in cardio-respiratory capacity expressed by the relative distance covered by players in Yo-Yo intermittent test level 1.

Experimental approach to the problem
Regarding match locomotion, Santana et al. [17] analysed the match running performance in the German Bundesliga before and after the COVID-19 lockdown. Their results suggest that teams covered a shorter total distance (TD) after the social distancing. However, the number of sprints performed by the players during the game did not change significantly. These findings were confirmed by Radzimiński et al. [18], who compared the physical match performance before and after the lockdown in the German Bundesliga and the Polish Ekstraklasa. They demonstrated that physical match activity in the Polish league was more influenced by the COVID-19 lockdown than the Bundesliga. The shorter period of typical soccer training before competition resumption applied in the Polish Ekstraklasa (only 18 days) was probably one of the reasons for these changes. Similarly, in the Spanish LaLiga soccer-specific team training was allowed only for 14 days before league renewal [19]. As a result, significant decreases in such physical performance variables as total distance, high-speed running distance, sprinting and number of high-intensity actions were noted during the post-lockdown phase of the season. Effect of COVID-19 lockdown on soccer match performance aerobic exercises on the stationary bike and strength training (using body weight and small weights). These training sessions were performed 4-6 times per week. The largest restrictions were enforced by Polish authorities in April, when physical activities were prohibited even outside (in the parks or forest). At the beginning of May, after more than seven weeks, players were allowed to train in small groups consisting of 6-10 players. Such training sessions included numerous technical, soccer-specific drills mixed with conditioning exercises and lasted about 60-75 minutes. Team training sessions restarted on 11 May, which was 18 days before competition resumption. Thus, the pandemic off-season period lasted 81 days [18]. The timeline containing the most important dates during the lockdown in the Polish Ekstraklasa is shown in Figure 1. None of the teams were allowed to play friendly games at this time. All these complications provided numerous difficulties for coaches and sport scientists in preparing players for the final part of the season.

Participants
The sample was composed of 3137 individual match observations from 209 professional soccer players who participated in official games of the Polish Ekstraklasa in the 2019/2020 season. Data were collected only from outfield players (i.e., goalkeepers were excluded) who completed the entire match. Considering the effect of dismissals in match running performance [24], the matches that involved a player dismissal (i.e., red card) were excluded. Furthermore, only data from players who participated in at least one match before and after the COVID-19 lockdown were included in the final sample.
A total of 52 matches were removed due to a dismissal/s from the final sample. Subsequently, the number of observations for the playing position of goalkeeper was 488.
In line with previous studies [16], the playing position was cate- In order to guarantee player and team confidentiality, all data were anonymized in accordance with the Declaration of Helsinki. This study was conducted in compliance with the local Board of Ethics (agreement number: 12/2021).

Variables and procedures
Match running performance data were collected using the computerised multiple-camera optical tracking system TRACAB (Chryron-Hego VID, New York, NY) with sampling frequency of 25 Hz. The validity and reliability of this video tracking system have recently been established [25]. In accordance with previous research, match running performance was classified into the following categories: total distance covered (TD), walking (0.7-7.2 km • h -1 ), jogging The influence of situational variables on match running performance has been well documented [26]. In this regard, the situational variables included in the present study were: match location (home or away), quality of opposition, and match result (win, draw, or lose).
In line with previous studies [26], the quality of opposition was defined as the difference in final ranking between teams at the end of the season analysed. All these contextual variables were taken into consideration in creating the model structure.

DISCUSSION
The current study aimed to establish possible changes in physical match performance in professional soccer players after the COVID-19 lockdown and indicate which playing positions were most affected.
In agreement with previous scientific literature [17][18][19]23], we players trained in small groups or performed home-based training following the recommendations given by medical committees and scientific evidence [11,29]. Therefore, negative physical match performance changes after such an in-season break could be expected [23]. The effective playing time was previously identified as one of the factors that should be taken into account when analysing the physical match performance in elite soccer [30]. Thus, eventual reduction in E t could be one of the reasons for lower physical match activity after season resumption. Nevertheless, despite the significant decrease in total match duration, the E t remained stable across pre-and post-lockdown periods (55.9 and 56.3 minutes respectively). Therefore, it can be assumed that E t did not affect the physical match performance during 11 final matches of the analysed season.
Most of the studies investigating physical performance in professional soccer matches usually monitor total distance and distance covered with high intensity, as these variables are considered to be the most important ones [31,32]. A relatively large number of high-intensity actions requires adequately long recovery periods, during which players try to avoid redundant activity.
Walking is an effective movement pattern that allows for fast recovery between high-intensity efforts. As a result, increased frequency of high-intensity actions may require longer distance covered in walking. Significantly longer walking distance covered by the players after the COVID-19 lockdown was observed, while jogging, running and TD decreased significantly. This effect was reported for all the playing positions. Probably maintaining the level of high-intensity performance (e.g. sprinting distance, NHIR) was compensated by a large number of activities with very low intensity. The simultaneous decrease in jogging, running and TD could possibly be caused by a lower level of soccer specific fitness.
As a result, players after the lockdown might need more time for recovery between high-intensity actions.
According to Guerrero-Calderón [12], potential muscle disorders generated during the quarantine might affect the ability to perform high-intensity activities and repeated sprints. This assumption was partly confirmed in this study. The present results showed a decline in distance covered in HSR for all playing positions, except ED. Additionally, the effects of the COVID-19 in-season break showed a decrease in distance covered in HIR and NHIR for CM and FW. Meanwhile, the distance covered in sprinting did not change significantly for any of the playing positions. It should be taken into account that these declines in match running performance showed trivial-to-small effects. Although players' aerobic capacity was not measured in the current study, this fitness component was previously shown to increase due to home-based training strategies during the lockdown [13][14][15]. In addition, some sports scientists and strength and conditioning coaches emphasized that quarantine caused by pandemic lockdown would lead to a decrease in the ability to generate Biology of Sport, Vol. 39 No4, 2022 1093 Effect of COVID-19 lockdown on soccer match performance disturbed the home advantage phenomenon [35]. Thus, investigating whether this turnover concerned physical match performance as well seems to be an important research topic.

CONCLUSIONS
The results of the current research indicate that the implemented lockdown significantly reduced match running performance in elite soccer players participating in the Polish Ekstraklasa league. However, the reduction in high-intensity efforts was not equal for all the positions. Moreover, distance covered in sprinting after the lockdown did not change significantly in comparison with the pre-lockdown period. Analysing the influence of such unexpected off-season periods is an important topic and provides practical information for soccer coaches. It is widely known that aerobic capacity is crucial for adequate match performance. Reductions in most of the locomotor variables could suggest an insufficient level of this fitness component after the COVID-19 lockdown. Additionally, shorter distance covered in HSR and lower NHIR could be caused by disturbed recovery ability, which is related to aerobic capacity as well. Therefore, an individualized approach in developing the ability to recover between high-intensity actions should be applied by coaches during home-based training.
Moreover, the differences according to playing position underline the necessity of training individualization regarding physical, technical and tactical demands. Fitness coaches should take into account the specific nature of the movement patterns for players from different positions when preparing exercises developing their physical fitness.